Recently, as the miniaturization and weight reduction of electronic products, electronic devices, communication devices, and the like are rapidly progressing and the need for electric vehicles has been greatly increased in relation to environmental problems, there is also a growing demand for performance improvements in secondary batteries used as power sources for these products. Among them, the lithium secondary battery has been attracting considerable attention as a high-performance battery because of its high energy density and high standard electrode potential.
The lithium-sulfur (Li—S) battery is a secondary battery using a sulfur-based material having an S—S bond (sulfur-sulfur bond) as a positive electrode active material and using lithium metal as a negative electrode active material. The Lithium-sulfur battery has advantages in that sulfur, which is the main material of the positive electrode active material, is very rich in resources, is not toxic, and has a low atomic weight. In addition, theoretical discharge capacity of the lithium-sulfur battery is 1675 mAh/g-sulfur, and its theoretical energy density is 2,600 Wh/kg. Since the energy density of the lithium-sulfur battery is much higher than the theoretical energy density of other battery systems currently under study (Ni-MH battery: 450 Wh/kg, Li—FeS battery: 480 Wh/kg, Li—MnO2 battery: 1,000 Wh/kg, Na—S battery: 800 Wh/kg), the lithium-sulfur battery is the most promising battery among the batteries developed so far.
During the discharging of the lithium-sulfur battery, an oxidation reaction of lithium occurs at the negative electrode (Anode) and a reduction reaction of sulfur occurs at the positive electrode (Cathode). Sulfur before discharging has an annular S8 structure. During the reduction reaction (discharging), as the S—S bond is cut off, the oxidation number of S decreases, and during the oxidation reaction (charging), as the S—S bond is re-formed, electrical energy is stored and generated using an oxidation-reaction reaction in which the oxidation number of S increases. During this reaction, the sulfur is converted from the cyclic S8 structure to the linear structure of lithium polysulfide (Li2Sx, x=8, 6, 4, 2) by the reduction reaction and eventually, when the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is finally produced. By the process of reducing to each lithium polysulfide, the discharging behavior of the lithium-sulfur battery is characterized by a step-wise discharge voltage unlike lithium ion battery.
Among lithium polysulfides such as Li2S8, Li2S6, Li2S4 and Li2S2, particularly, lithium polysulfide (Li2Sx, usually x>4), which has a high oxidation number of sulfur, is easily dissolved in a hydrophilic electrolyte solution. The lithium polysulfide dissolved in the electrolyte solution diffuses away from the positive electrode where the lithium polysulfide is generated by the concentration difference. Thus, the lithium polysulfide eluted from the positive electrode is lost to the outside of the positive electrode reaction zone, making it impossible to perform the stepwise reduction to lithium sulfide (Li2S). That is, since the dissolved lithium polysulfide which is present in the dissolved state away from the positive electrode and the negative electrode cannot participate in the charging and discharging reactions of the battery, the amount of sulfur involved in the electrochemical reaction is reduced at the positive electrode and as a result, it is a major factor in reducing the charging capacity and energy of the lithium-sulfur battery.
Furthermore, in addition to being floated or deposited in the electrolyte solution, the lithium polysulfide diffused into the negative electrode reacts directly with lithium and sticks to the surface of the negative electrode in the form of Li2S, thus causing the corrosion of the lithium metal negative electrode.
In order to minimize the elution of lithium polysulfide, a large number of studies are underway. For example, the studies which use carbon containing nitrogen or oxygen, known as adsorbents of lithium polysulfide or similarly which induce the adsorption of lithium polysulfide by coating or adding polymer to electrode or composite have been carried out, but it is not enough to completely solve the dissolution problem of lithium polysulfide.